ANDROID: ashmem_rust: add toggle for PROT_EXEC

The memfd type simulates SET_PROT_MASK using file seals, but there's no
file seal to prevent mapping executable memory. Thus, add a toggle that
removes the ability to unset the PROT_EXEC bit on an ashmem file.

Bug: 370906207
Change-Id: Ia90c2cfca689d152fb1765f2abcf6bb3f60cd05d
Signed-off-by: Alice Ryhl <aliceryhl@google.com>

drivers/staging/android/ashmem_rust.rs

@@ -50,7 +50,7 @@ mod shmem;
 use shmem::ShmemFile;
 
 mod ashmem_toggle;
-use ashmem_toggle::{AshmemToggleMisc, AshmemToggleRead, AshmemToggleShrinker};
+use ashmem_toggle::{AshmemToggleExec, AshmemToggleMisc, AshmemToggleRead, AshmemToggleShrinker};
 
 /// Does PROT_READ imply PROT_EXEC for this task?
 fn read_implies_exec(task: &Task) -> bool {
@@ -67,6 +67,7 @@ fn has_cap_sys_admin() -> bool {
 
 static NUM_PIN_IOCTLS_WAITING: AtomicUsize = AtomicUsize::new(0);
 static IGNORE_UNSET_PROT_READ: AtomicBool = AtomicBool::new(false);
+static IGNORE_UNSET_PROT_EXEC: AtomicBool = AtomicBool::new(false);
 
 fn shrinker_should_stop() -> bool {
     NUM_PIN_IOCTLS_WAITING.load(Ordering::Relaxed) > 0
@@ -84,6 +85,7 @@ struct AshmemModule {
     _misc: Pin<Box<MiscDeviceRegistration<Ashmem>>>,
     _toggle_unpin: Pin<Box<MiscDeviceRegistration<AshmemToggleMisc<AshmemToggleShrinker>>>>,
     _toggle_read: Pin<Box<MiscDeviceRegistration<AshmemToggleMisc<AshmemToggleRead>>>>,
+    _toggle_exec: Pin<Box<MiscDeviceRegistration<AshmemToggleMisc<AshmemToggleExec>>>>,
 }
 
 impl kernel::Module for AshmemModule {
@@ -108,6 +110,7 @@ impl kernel::Module for AshmemModule {
             )?,
             _toggle_unpin: AshmemToggleMisc::<AshmemToggleShrinker>::new()?,
             _toggle_read: AshmemToggleMisc::<AshmemToggleRead>::new()?,
+            _toggle_exec: AshmemToggleMisc::<AshmemToggleExec>::new()?,
         })
     }
 }
@@ -348,6 +351,11 @@ impl Ashmem {
             prot |= asma.prot_mask & PROT_READ;
         }
 
+        if IGNORE_UNSET_PROT_EXEC.load(Ordering::Relaxed) {
+            // Add back PROT_EXEC if asma.prot_mask has it.
+            prot |= asma.prot_mask & PROT_EXEC;
+        }
+
         // The user can only remove, not add, protection bits.
         if (asma.prot_mask & prot) != prot {
             return Err(EINVAL);

drivers/staging/android/ashmem_toggle.rs

@@ -4,7 +4,7 @@
 
 //! Provides knobs for Rust ashmem.
 
-use crate::{ashmem_range, IGNORE_UNSET_PROT_READ};
+use crate::{ashmem_range, IGNORE_UNSET_PROT_EXEC, IGNORE_UNSET_PROT_READ};
 use core::{marker::PhantomData, sync::atomic::Ordering};
 use kernel::{
     c_str,
@@ -92,3 +92,16 @@ impl AshmemToggle for AshmemToggleRead {
         IGNORE_UNSET_PROT_READ.load(Ordering::Relaxed)
     }
 }
+
+pub(crate) struct AshmemToggleExec;
+
+impl AshmemToggle for AshmemToggleExec {
+    const NAME: &'static CStr = c_str!("ashmem_ignore_unset_prot_exec");
+    fn set(enabled: bool) -> Result<()> {
+        IGNORE_UNSET_PROT_EXEC.store(enabled, Ordering::Relaxed);
+        Ok(())
+    }
+    fn get() -> bool {
+        IGNORE_UNSET_PROT_EXEC.load(Ordering::Relaxed)
+    }
+}